Since nanometer-size particles have physical properties that metallic chips do not have, the studies on nanometer-size particles have been advancing recently. Although a vapor-phase oxidation method has been available for the synthesis of nanometer-size particles, it is difficult to collect the resulting nanometer-size particles by this method. Hence, techniques for synthesizing nanometer-size particles utilizing plasma in liquid are set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2008-13810 (i.e., Patent Literature No. 1) and Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2010-121193 (i.e., Patent Literature No. 2), for instance.